Leaving aside wood, which is rarely examined as to its value as a com-
bustible, the fuels used industrially are mainly coal, charcoal and mineral
oils ; the last will be considered later. The coals are distinguished according
to the degree of carbonisation as peat, lignite, bituminous coal and anthracite.
Extensive use as a fuel is also made of coke, the residue of the dry distillation
of bituminous coal, the volatile products being illuminating gas, ammonia
and tar. Fuel-blocks (briquettes) are also largely used at the present time ;
these are obtained by the compression in moulds of fragments of different
coals, usually with the addition of cementing materials. In this way waste
coal may be utilised and coal which is inconvenient to use on account of
its physical condition rendered more useful.
The quality and technical value of coal are determined by chemical
analysis and calorific examination. The methods adopted are the same
for all coals and are described below under the heading : General Methods.
In the special part data will then be given relating to each of the different
types of coal.
In all cases, selection of the sample is of great importance.
Sampling.óCoal is usually far from homogeneous, and care must be
taken that the sample for analysis represents as closely as possible the mean
composition of the whole of the parcel to be examined. When the sample
is to be taken from a mine or from a large quantity, portions are taken with
a shovel from different, regularly distributed points of the mass, a large
quantity being thus collected. The larger lumps of this are broken up and
the whole well mixed and spread out in the form of a square and the diagonals
of the latter drawn. Two opposite triangles are then discarded and the
remaining two, further disintegrated and mixed, formed into another square.
These operations are repeated several times until a sample of about 2 kilos
is obtained, this being reduced to small pieces and stored in dry, tightly
closed vessels. When, however, the laboratory is supplied with a limited
sample, the whole of the latter is broken up and stored as above.
In either case, a portion of the sample thus prepared sufficient for the
determinations to be made (about 200 grams) is reduced to coarse powder
and stored separately in a dry, air-tight vessel. For each single determina-
tion, part of this sample is powdered to the degree of fineness requisite in
each case, care being taken not to throw away any part. Consequently,
when the portion taken has been powdered and sieved through the sieve
297eveals numerous minute fissures which have been made in the oxide during